1. Field of the Invention
This invention relates to a magnetic playback signal channel, and in particular to equalization of a signal channel utilizing a magnetoresistive playback head.
2. Description of the Prior Art
The invention, as well as the prior art, will be described with respect to the figures, of which:
FIG. 1 is a plot of magnetoresistive head wavelength responses known in the prior art.
FIG. 2a is a plot of the time response of an unshielded magnetoresistive head of an isolated pulse and an ideal equalized isolated pulse of the prior art.
FIG. 2b illustrates the recording current waveform for two contiguous "1's" of a (O, k) modulation code,
FIG. 3 illustrates waveforms of a two channel detector known in the prior art,
FIG. 4 illustrates apparatus used in the practice of the present invention,
FIG. 5 is a plot of the response of an unshielded magnetoresistive head to a dipulse,
FIG. 6 is a plot of waveforms useful in understanding the present invention,
FIG. 7 is a plot of waveforms useful in the design of the equalizer of the invention,
FIG. 8 is a block diagram of apparatus for reproducing magnetically recorded signals in accordance with the invention,
FIG. 9 is a schematic drawing of an equalizer in accordance with the invention, and
FIG. 10 is a plot of the response of a single channel detector utilizing the equalizer of the invention for a charge constrained (d,k) code.
The magnetoresistive (MR) reproduce head is well known in the art. Its high signal output, excellent short wavelength response, and ease of manufacture by batch fabrication techniques have made it an attractive candidate for use in modern digital data storage devices. Since its disclosure in U.S. Pat. No. 3,493,694, issued in the name of R. P. Hunt, the unshielded MR head has been improved and elaborated upon; U.S. Pat. Nos. 3,840,898 and 4,987,508 reflect two of such improvements and elaborations.
Referring to FIG. 1, it is seen that the unshielded MR head response 11 has considerably higher maximum output 10 at long wavelengths than the maximum output 10' of an equivalent shielded MR head. This occurs in the unshielded head because the flux from a long wavelength signal transition influences the MR element across its entire height, while the flux from a short wavelength signal transition is intercepted at the element only in the region of the head to tape interface. As current applications of the unshielded MR head are typically for short wavelength use, i.e. 80 kfci and above, and because difficulty was generally experienced in the prior art in equalizing the unshielded MR head over the broad signal frequency range characteristic of the head, the shielded MR head was developed wherein shields were added on either side of the MR element to reduce the sensitivity of the MR head to long wavelength signals. Thus for the shielded MR head, decreased difficulty in equalization was attained at the expense of increased fabrication complexity when compared to the unshielded MR head.
In modern magnetic recording systems, data is often encoded using run length limited codes, generically designated as (d,k) codes. They are extensions of earlier non return to zero modulation codes where binarily recorded "zeros" are represented by no flux change in the magnetic medium, while binary "ones" are represented by transitions from one direction of recorded flux to the opposite direction. In a (d,k) code, the above recording rules are maintained with the additional constraints that a least d "zeros" are recorded between successive "ones", and no more than k "zeros" are recorded between successive "ones". The first constraint arises to obviate intersymbol interference occurring due to pulse crowding of the reproduced transitions when "ones" are contiguously recorded. The second constraint arises in recovering a clock from the reproduced data by "locking" a phase lock loop to the reproduced transitions. If there is too long an unbroken string of contiguous "zeros" with no interspersed "ones" transitions, the clock generating phase locked loop will drop out of synchronism. For example, in a (1,7) code there is at least one "zero" inserted between the recorded "ones", and there are no more than seven recorded contiguous "zeros" between recorded "ones". It will be appreciated that runs of seven "zeros" in the recorded patterns can contribute a significant direct current (d.c.) component to the (1,7) code.
The paper entitled "Signal Processing In Recording Channels Utilizing Unshielded Magnetoresistive Heads", N. L. Koren, IEEE Trans Magn., vol MAG-26, no. 5, 1990, p. 2166 discloses a procedure for equalizing and detecting signals from an unshielded MR head in a system utilizing a (1,7) code. This equalization procedure requires measurement of the head playback response to a recorded isolated pulse. The excellent low frequency response of the unshielded MR head makes this head susceptible to saturation during playback of an isolated pulse due to the long wavelength component in such a pulse. Therefore, the response of the unshielded MR head cannot be measured by recording and playing back an isolated pulse in a straightforward manner. Additionally, the modulation code used for data recording must also be essentially d.c. free to preclude head saturation during data playback. Hence, some means is required to effectively remove the d.c. component either from the isolated pulse or from the modulation code. Write equalization, known in the art wherein high frequency pulses are added to the write current, was utilized to eliminate the d.c. component in the isolated pulses. This provided a measurable played back isolated pulse, and allowed implementation of the procedure for designing the equalizer and detector as further disclosed in the referenced paper. It also ensured saturation-free playback of the encoded data patterns by removal of the d.c component of the code at the MR head.
While the unequalized response of an unshielded MR head to an isolated pulse cannot be directly measured due to saturation effects, as stated above, the general shape of the time domain response curve may be inferred from its frequency response. FIG. 2a shows the broad skirts 14,14' of the unshielded MR head time response 16 for an assumed isolated pulse input. These characteristic skirts arise due to the excellent low frequency response of the unshielded MR head. In the teachings of the prior art, the prior art equalizer slimmed down the isolated pulse response 16 to provide a unidirectional, time limited, output pulse 18. To minimize intersymbol interference, the width of the equalized isolated pulse response of the prior art 18 was well contained in the bit interval equal to approximately twice the system minimum transition time, Tmin. As shown in FIG. 2b, Tmin is the time interval between two adjacent "1"s in the recorded waveform 15. Thus, the prior art teaches signal equalization by confining the energy of the reproduced pulse 16 to provide the slimmed, equalized pulse 18.
In view of the above, it will be appreciated that in a (d,k) code where a change in flux represents a "one", that each change of flux in the coded pattern gives rise to an output pulse of alternating polarity whose shape is similar to the equalized pulse 18 of FIG. 2a. These pulses have conventionally been detected by means of a two channel detector. The two channel detector, well known in the prior art, comprises a zero crossing peak detector and an amplitude qualifier. Referring to FIG. 3 for the played back waveforms of a typical two channel detector, an equalized bi-directional played back signal 20 is clipped at levels +Vc, 22, and -Vc,24, and the resultant signal is rectified to provide the signal 26. The signal 26 is differentiated resulting in the signal 28. It will be noted that the zero crossings of the signal 28 (e.g. 30,32,34) correspond to the peaks 36,38,40 of the original signal 20. This occurs in one channel of the two channel detector. At the times of the true zero crossings, 30, 32,34, synchronized clock signals 37,39,41 test the signal 26 to ensure the presence of the peaks 36',38' 40'. This occurs in the second channel of the two channel detector. The presence of the peaks 36',38',40' at the zero crossing times 30,32,34 allows the corresponding clock pulses 37,39, 41 to generate the output signals 43,45,47. However there are other zero crossings associated with the signal 28 which do not correspond to peaks in the original signal 20. One type of such "false" zero crossings correspond to "shoulders" (e.g. 44) in the original signal 20 which do not designate true data signals. The typical shoulder 44 results from the prior art equalized signal (18) which rapidly decays to the system baseline when there is an interval (i.e., several "0's") between played back flux changes (i.e. "1's"). The zero signal 42 in the waveform 28 does not result in an output signal occurring at clock time 35, because there is no peak signal in waveform 26 at clock time 35. (It will be noted that the signals 42',42",42"' are at the baseline due to the clipping of the original signal 20. However, they do not result in false read out signals as there are no clock signals occurring at the corresponding times.)